1. Field of the Invention
The present invention relates wireless communications, and in particular to a system and method for optimizing spectral efficiency when supporting mobile devices that have a wide range of mobility speeds.
2. Description of the Related Art
Wireless communication systems such as Orthogonal Frequency-Division Multiple Access (“OFDMA”) systems typically support mobile devices having wide ranges of speeds. These mobile devices communicate with fixed base stations that are, in turn, directly or indirectly coupled to other communication systems, e.g., the Internet, other carriers, etc. Certain mobile devices may be moving at high rates of speed, such as cell phones in a moving automobile, while other devices may be moving slowly or not at all, e.g., a phone being used while walking or stationary.
Design parameters of systems such as OFDMA systems include sub-carrier separation/spacing and OFDMA symbol duration. Typically, the design of OFDMA systems is targeted toward the worst case, namely the highest speed mobile devices, expected in the system. As such, system designers allow a large enough sub-carrier space to reduce inter-carrier-interference (“ICI”) caused by the Doppler effect of the targeted highest mobility speed and frequency offset. This leads to inefficient spectral usage when the system is serving a wide range of mobility speeds.
The inefficiency described above results from the need to use larger sub-carrier spacing than is necessary to support lower speed devices. A result is a lower number of usable sub-carriers. In addition, there is inefficiency because the system design results in a higher cyclic prefix (“CP”) ratio (CP/OFDM symbol duration) and higher pilot overhead than is necessary for the lower speed devices. Examples are shown in FIGS. 1 & 2. FIG. 1 shows a graph 2 of a relationship between power density and frequency for various mappings of symbols to sub-carriers. Each band shown in FIG. 1 represents a modulation symbol to sub-carrier mapping. It is readily observed that across a given frequency band, large sub-carrier spacing is required. In addition, as is shown in graph 4 in FIG. 2, convention methods necessitate a large cyclic prefix (“CP”) 6 overhead in the time domain as compared with the temporal portion usable for the transmission of user data 8.
A possible solution is to divide resources in a given transmission time interval (“TTI”), i.e., scheduling interval, into two parts with each part using different design parameters. One part (with a larger sub-carrier space) would support high speed mobile devices and the other part (with a smaller sub-carrier space) would support slower speed mobile devices. However, this arrangement is unnecessarily over-complicated because two different sets of design parameters need to be used, e.g., sampling rate, size of fast fourier transform (“FFT”), etc. Accordingly, it is desirable to have a method and system that can maintain high spectral efficiency, i.e., reduce the CP/OFDM symbol duration ratio, regardless of the speed of the mobile device. It is also desirable that this method and system not require multiple sets of design parameters and allows scalable resource assignment for different speeds mobile devices for both uplink (mobile device to base station) and downlink (base station to mobile device) communications.